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444                             BOILED LINSEED OIL

If the oil is distilled in a current of steam, the distillate will contain any
oil of- turpentine or benzine present.

The unsaponifiable substances may be determined quantitatively by
the methods described on p. 388.

4.  Detection of Resin.—The oil is shaken with an equal volume of
about 70% alcohol, the alcoholic liquid being then separated and evaporated,
and the residue tested by means of Morawski's reaction (see p. 390).

Oils boiled with resinates naturally contain small quantities of resin
due to the dryer. Addition of resin should be assumed only when the
alcoholic extract is large in amount and the acid number of the oil is high
(at least above 12 ; see later).

5.  Detection of Siccatives.—This may be made on the ash of the oil
or by dissolving the oil in ether and shaking with dilute nitric acicl, the
nitric acid solution being tested by the ordinary methods for lead, man-
ganese, calcium, zinc and cobalt.

6.  Drying Properties.—A drop of the oil is spread uniformly on a
glass plate (5 X 10 cm.) and left in the air, protected from direct sunlight,
at 15-20°.   From time to time the course of the drying is tested by pressing
the oily layer hard with the finger ;   drying is complete when the finger
no longer adheres to the surface.   The latter is then heated to 100° to see
if cracking occurs.

Further, 25 parts of the oil are mixed intimately with 20 parts of zinc
white or minium, the paste thus obtained being spread uniformly on an
iron plate and left to dry as before.

Good boiled linseed oil should satisfy the following requirements :

That boiled at a moderate temperature should be only slightly coloured

and should yield a perfectly white paste when mixed with pure lead or zinc

white.

Its specific gravity should not be below 0-935 an(l as a ni'e n'cs between
0-935 and °'94S, but oil heated at a very high temperature (double boiled,
burned) may have a value as high as 0-99. The presence of extraneous oils
(vegetable, animal, mineral) lowers the specific gravity.

The iodine number may vary from 150 to 172 in light boiled oils, but may
fall to 70°, with those strongly boiled and dense.

The saponifi cation number should lie between 190 and 195, and is lowered
by colza oil and especially by mineral or resin oil.

The acid number should not exceed 12 (usually it stands between 7 and 10)
and is raised by free resin.

It should not contain foreign oils or free resin.

The content of unsaponifiable substances should not exceed 2% at the most.

The amount of dryer should be such that the oil does not leave more than
i% of ash.

In the drying test, pale oils should diy completely within 20 hours and dark
ones within 12 hours. After being heated at 100°, the dry layer should not
exhibit cracking and should become detached in scales when scraped with a
knife. When mixed with zinc white or minium, it should dry completely within
24 hours. .     .     .     .115       0-926—0-970 j      81—83     \     80— 81